With expedite development of information technologies, two-dimensional fundamental geometric information data can hardly satisfy the requirements of urban information construction, and three-dimensional digital city models have become an important orientation for studies of urban geographical information systems. At present, to build a three-dimensional digital city model, the geometric modeling dominantly adopts an oblique photogrammetric technology to address issues such as lack of universality of the data of vertical orthoimages with satellite images and ground measurement equipment, and complex post-processing of data. Oblique photogrammetry is a technology of capturing images simultaneously from different angles such as vertical and oblique by a plurality of airborne cameras so to obtain more complete and accurate information of ground objects. Compared with traditional manual modeling, a three-dimensional model generated thereby has a shorter modeling period, a lower cost, and a higher accuracy. The three-dimensional modeling of oblique photogrammetry may quickly recover the real scene, which is undoubtedly advantageous over the traditional vertical aerial photography; it not only effectively makes up for the shortfalls of traditional orthoimages, but also may enhance data access efficiency of the quick three-dimensional modeling technology. Currently, oblique photogrammetry is still carried out by a large aircraft, which obviously has disadvantages such as bulkiness and inflexibility. An airborne photogrammetric system is a more ideal and easily popularized oblique photogrammetry approach due to its characteristics such as low altitude, convenience, flexibility, and high image resolution.
The airborne oblique photogrammetry technology has the following characteristics: 1) reflecting real conditions surrounding a ground object: compared with vertical orthoimages, oblique images enable a user to view the ground object from multiple angles, which reflects the actual conditions of the ground object in a more realistic way and significantly makes up for the deficiencies of vertical orthoimage-based applications; 2) capability of realizing single image measurement: with application of support software, the oblique images may be directly applied to measure height, length, area, angle, and slope, etc., which expands industrial applications of the oblique photogrammetry technology; 3) capability of acquiring profile textures of buildings: for various kinds of three-dimensional digital city applications, the city three-dimensional modeling costs may be effectively reduced by leveraging the characteristics of large-scale imaging of aerial photography in addition to extracting and pasting textures in batch from the oblique images; 4) small data amount easing network release: compared with the bulky three-dimensional data of the three-dimensional GIS technology, the data amount of the images acquired using the oblique photogrammetry technology is far less, and the image data format may be quickly released to the network using a mature technology to realize sharing and application.
A traditional airborne oblique photogrammetry shoots a ground object in various angles through 5 cameras arranged according to certain mount angles to thereby obtain true three-dimensional image data of the ground object; in the current unmanned aircraft market, the lightest 5-piece cameras is above 5 kg; for a mini unmanned aircraft with a maximum takeoff weight of only 20-30 kg, the cameras are too heavy, bulky and costly, which adds the difficulty to application. Therefore, it is desired that the unmanned aircraft is capable of carrying different number of capturing cameras in different application scenarios. In other words, the number of oblique photogrammetric cameras carried by a camera hanger of the unmanned aircraft is scalable as needed. Additionally, at different flight heights, the optimum oblique photogrammetric angles required by the oblique cameras carried by the unmanned aircraft are also different; therefore, it is desired that a camera hanger on the unmanned aircraft is enabled for appropriate adjustment of capturing angles of the oblique cameras. Further, not only the firmness of connecting the cameras to the chassis of the unmanned aircraft is considered, vibrations caused to the unmanned aircraft due to factors such as air current during flight of the unmanned aircraft should also be considered. Therefore, it becomes an imminent problem currently to solve how to build a camera mounting apparatus for airborne oblique photogrammetry use, which is simple and light-weighted, scalable with respect to the number of cameras, adjustable with respect to tilt angles of oblique cameras, and capable of mounting the cameras securely.